Variable gain amplification for linearization of nmr signals

ABSTRACT

Various approaches of amplifying an NMR signal in response to a transmitted NMR pulse include estimating the characteristic time associated with the NMR signal; transmitting the NMR pulse to the sample and receiving the NMR signal therefrom; and applying a time-dependent amplifier gain to the received NMR signal based at least in part on the estimated characteristic time.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of, and incorporatesherein by reference in its entirety, U.S. Provisional Patent ApplicationNo. 62/434,132, which was filed on Dec. 14, 2016.

FIELD OF THE INVENTION

The field of the invention relates, generally, to nuclear magneticresonance (NMR) systems and, more particularly, to systems and methodsfor providing variable gain amplification of NMR signals.

BACKGROUND

Nuclear magnetic resonance (NMR) is a well-known analytic technique thathas been used in a number of fields, such as spectroscopy, bio-sensingand medical imaging. In general, an NMR device includes transceivercircuits to transmit signals to a tested sample and receive echo signalstherefrom. The echo signals are then analyzed to obtain imaging and/ormaterial information of the sample. The NMR echo signals, however, aretypically very small (on the order of microvolts) and represent freeinduction decay; this can pose a considerable challenge to the NMRsystem. For example, the sensitivity of the NMR transceiver has to besufficient in order to detect the small echo signals. In addition, theNMR transceiver has to amplify the received NMR signals to a levelsufficient to permit processing for analysis.

Conventionally, the NMR transceiver includes an amplifier of a fixedgain for amplifying the detected NMR signals. Fixed-gain amplification,however, may not be sufficient to amplify the damped portions of the NMRdecay signals to a desired level suitable for signal processing. Inaddition, fixed-gain amplification may result in saturation of theless-damped portions of the NMR signals, thereby providing inaccurateNMR analysis. Accordingly, there is a need for an approach that ensuresthe detected NMR echo signals, particularly the damped portions, areamplified to a level sufficient to permit analysis without saturation ofthe less-damped portions of the NMR echo signals.

SUMMARY

Embodiments of the present invention provide systems and methods forapplying a time-dependent amplifier gain to an NMR signal having a freeinduction decay. In various embodiments, the decay signals have dampedand less-damped portions (as defined below), and the time-dependent gainamplifies the damped portion of the decay signal to a greater degreethan the less-damped portion. For example, the time-dependent amplifiergain may be a gradually (e.g., exponentially) increasing gain. As aresult, the damped portion is amplified to a desired level (e.g., on theorder of millivolts) suitable for signal processing and analysis, whilethe less-damped portion is amplified to a lesser degree so as to avoidsignal saturation. In one embodiment, after the NMR signal is processedand transmitted to a controller in the NMR system, the controllercomputationally applies to the amplified signal a gain function that isinverse to the applied amplifier gain, and then analyzes the resultingsignal to obtain information (such as the characteristic time and/orLarmor frequency) associated with the NMR signal; this information maybe used, for example, to facilitate chemical composition analysis,medical imaging, and/or bio-sensing.

In some embodiments, the time-dependent amplifier gain may have aprofile different from the gradually increasing exponent. For example,the amplifier gain may be increased in a step-wise manner, and theoverall profile of the discrete gains is approximated as an exponentialincrease; this may advantageously lessen the design burden of amplifierstages implemented in the NMR system. In addition, the amplifier gainapplied to the received NMR signals may be dynamically adjusted duringNMR measurements. For example, the characteristic of the time-dependentamplifier gain (such as the time constant of the exponential gain or thediscrete increase of the gain) applied to the currently received signalmay be determined based on the characteristic time(s) of the previouslyreceived NMR signal(s). This approach may advantageously enable thedamped portions of the received NMR echo signals to be amplified to agreater degree (compared with fixed-gain amplification) based on theactual, “real-time” measurements of the sample, providing greateraccuracy and/or higher resolution.

Accordingly, in one aspect, the invention pertains to a method ofamplifying an NMR signal in response to a transmitted NMR pulse. Invarious embodiments, the method includes estimating a characteristictime associated with the NMR signal; transmitting the NMR pulse to asample and receiving the NMR signal therefrom; and applying atime-dependent amplifier gain to the received NMR signal based at leastin part on the estimated characteristic time. In one implementation, thecharacteristic time is estimated based on one or more previous NMRmeasurements of the sample. In addition, the method may further includecomputing the second characteristic time associated with the amplifiedNMR signal. For example, the second characteristic time may be computedby applying an inverse gain function to the amplified NMR signal. Insome embodiments, the method further includes analyzing at least aportion of the amplified NMR signals occurring after passage of thesecond characteristic time.

In addition, the method may further include transmitting the second NMRpulse to the sample and receiving the second NMR signal therefrom; andapplying the second time-dependent amplifier gain to the second NMRsignal based at least in part on the second characteristic time. In oneembodiment, the time-dependent amplifier gain includes an exponentialgain profile. The time constant of the exponential gain profile and thecharacteristic time may be within an order of the magnitude. In anotherembodiment, the time-dependent amplifier gain includes a series ofdiscrete gain increases. The profile of the series of discrete gainincreases may include an exponent.

In another aspect, the invention relates to an NMR apparatus including atransceiver for transmitting an NMR pulse to a sample and receiving anNMR signal therefrom; and a controller configured to (i) estimate acharacteristic time associated with the NMR signal; (ii) cause thetransceiver to transmit the NMR pulse to the sample and receive the NMRsignal therefrom; and (iii) cause a time-dependent amplifier gain to beapplied to the received NMR signal based at least in part on theestimated characteristic time. In addition, the NMR apparatus mayfurther include a variable-gain amplifier for applying thetime-dependent amplifier gain to the received NMR signal. In oneimplementation, the variable-gain amplifier is a programmable-gainamplifier.

In some embodiments, the NMR apparatus further includes a pulse-sequencegenerator having a gain setting associated with gains generated by thevariable-gain amplifier. The pulse-sequence generator may generatemultiple NMR pulses; the amplitude difference between adjacent pulsesmay be determined based at least in part on a step size of the gainsgenerated by the variable-gain amplifier. In addition, the controllermay be further configured to estimate the characteristic time based onone or more previous NMR measurements of the sample. In variousembodiments, the controller is further configured to compute the secondcharacteristic time associated with the amplified NMR signal. Inaddition, the controller may be further configured to computationallyapply an inverse gain function to the amplified NMR signal. In oneembodiment, the controller is further configured to analyze at least aportion of the amplified NMR signals occurring after passage of thesecond characteristic time.

In some embodiments, the controller is further configured to cause thetransceiver to transmit the second NMR pulse to the sample and receivethe second NMR signal therefrom; and cause the second time-dependentamplifier gain to be applied to the second NMR signal based at least inpart on the second characteristic time. In one embodiment, thetime-dependent amplifier gain includes an exponential gain profile. Thetime constant of the exponential gain profile may have the same order ofthe magnitude of the characteristic time. In another embodiment, thetime-dependent amplifier gain includes a series of discrete gainincreases. The profile of the series of discrete gain increases mayinclude an exponent.

Another aspect of the invention relates to a method of dynamicallyvarying gain amplification of received NMR signals in response totransmitted NMR pulses, each received NMR signal corresponding to atransmitted NMR pulse. In various embodiments, the method includes (i)transmitting the first one of the NMR pulses to a sample and receivingtherefrom the first one of the NMR signals; (ii) determining acharacteristic time associated with the first one of the NMR signals;(iii) transmitting the second one of the NMR pulses to the sample andreceiving therefrom the second one of the NMR signals; and (iv) applyinga time-dependent amplifier gain to the received second one of the NMRsignals based at least in part on the determined characteristic time.

In yet another aspect, the invention pertains to an NMR apparatusincluding a transceiver for transmitting NMR pulses to a sample andreceiving NMR signals therefrom, each NMR signal corresponding to atransmitted NMR pulse; and a controller. In various embodiments, thecontroller is configured to (i) cause the transceiver to transmit thefirst one of the NMR pulses to the sample and receive therefrom thefirst one of the NMR signals; (ii) determine a characteristic timeassociated with the first one of the NMR signals; (iii) cause thetransceiver to transmit the second one of the NMR pulses to the sampleand receive therefrom the second one of the NMR signals; and (iv) causethe transceiver to apply a time-dependent amplifier gain to the receivedsecond one of the NMR signals based at least in part on the determinedcharacteristic time.

In general, as used herein, the term “substantially” means ±10%, and insome embodiments, ±5%. In addition, reference throughout thisspecification to “one example,” “an example,” “one embodiment,” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present technology. Thus, the occurrences ofthe phrases “in one example,” “in an example,” “one embodiment,” or “anembodiment” in various places throughout this specification are notnecessarily all referring to the same example. Furthermore, theparticular features, structures, routines, steps, or characteristics maybe combined in any suitable manner in one or more examples of thetechnology. The headings provided herein are for convenience only andare not intended to limit or interpret the scope or meaning of theclaimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, with an emphasis instead generally being placedupon illustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIGS. 1A and 1B illustrate an exemplary NMR system in accordance withvarious embodiments;

FIGS. 2A and 2B depict amplifier gain applied to a received NMR signalin accordance with the prior art;

FIGS. 3A and 3B depict an exemplary time-dependent amplifier gainapplied to a received NMR signal in accordance with various embodiments;

FIG. 3C depicts an exemplary inverse gain function applied to anamplified NMR signal for analysis in accordance with variousembodiments;

FIGS. 4A and 4B depict another exemplary time-dependent amplifier gainapplied to a received NMR signal in accordance with various embodiments;

FIG. 4C depicts an exemplary inverse gain function applied to anamplified NMR signal for analysis in accordance with variousembodiments;

FIG. 5 depicts an approach for dynamically adjusting the amplifier gainapplied to the received signals during NMR measurements in accordancewith various embodiments;

FIG. 6A depicts an exemplary NMR transceiver in accordance with variousembodiments;

FIG. 6B depicts a pulse-sequence generator including settings of theamplifier gains to be applied to the NMR signals in accordance withvarious embodiments;

FIG. 7 is a flow chart illustrating an approach for dynamicallyadjusting the amplifier gain applied to the received signals during NMRmeasurements in accordance with various embodiments.

DETAILED DESCRIPTION

FIGS. 1A and 1B illustrate an exemplary NMR system 100 in accordancewith various embodiments of the present invention including an NMR coil102 surrounding a sample 104 being analyzed, a magnet 106 for generatinga static magnetic field across the sample 104 and the coil 102, an RFtransceiver 108 coupled to the NMR coil 102, and a controller 110 forcontrolling operation of the RF transceiver 108. In one implementation,the transceiver 108 includes a transmitter (Tx) portion 112 forgenerating and transmitting RF signals to the sample 104 and a receiver(Rx) portion 114 for receiving echo signals from the sample 104.

Referring to FIG. 1A, during NMR measurements, the magnet 106 isactivated to generate a substantially homogeneous magnetic field B₀across the sample 104; individual magnetic moments of the spins in thesample 104 may attempt to align with the applied field B₀. As a result,the magnetic moments of all the nuclei in the sample sum to a netmagnetic moment that precesses about the axis of the field B₀ at acharacteristic Larmor frequency, ω₀, satisfying ω₀=γ B₀, where γ is agyromagnetic ratio. Because different nuclei have different values ofthe gyromagnetic ratio, measuring the Larmor frequency of the sample 104allows material properties (e.g., the chemical composition) of thesample 104 to be revealed. In various embodiments, to observe precessionof the net magnetic moment, the controller 110 causes the transmitter112 to transmit an RF signal 116 (typically comprising a pulse sequence)having a resonant frequency substantially close (e.g., within ±10% or±5%) to the Larmor frequency ω₀ to the coil 102; the coil 102 thengenerates an RF magnetic field that causes the net magnetic moment ofthe nuclei in the sample 104 to be “tipped” away from the axis of thestatic field B₀. Typically, the RF magnetic field has a time-dependentmagnitude and is generated in a plane perpendicular to the axis of thestatic field B₀. Referring to FIG. 1B, after a predetermined timeduration, Δt, the transmitter 112 stops transmission of the RFexcitation signal 116, and the coil 102 is switched via, for example, amechanical switch or an electrical switch (e.g., a transistor) to thereceiver 114 for receiving the echo signals from the sample 104. Uponstopping the RF excitation, the nuclear spins within the sample 104precess around the B₀-axis at the Larmor frequency ω₀; this induces acorresponding signal oscillation. The nuclear spins then slowly losephase coherence via spin-spin interactions, which manifest themselves ina macroscopic average as an exponential relaxation or damping signal 118(referred to as “free induction decay”) in the precession of the netmagnetic moment. The oscillation and relaxation of the NMR signal can bedetected by the coil 102. Because the spin-spin interactions arepeculiar to the material of the sample 104 being tested, thecharacteristic time, commonly referred to as T₂, of the relaxationsignal is also material-specific. Thus, by measuring the Larmorfrequency ω₀ (e.g., for spectroscopy) and/or characteristic time T₂(e.g., for relaxometry), NMR techniques can be used as an analytic toolin a number of fields, including but not limited to chemical compositionanalysis, medical imaging, and bio-sensing. An exemplary NMR system isdescribed in U.S. Pat. No. 8,643,368, the entire disclosure of which ishereby incorporated by reference.

Typically, the detected NMR signals are very small (on the order of μV);therefore, it may be necessary to amplify the received signals to adesired level (e.g., on the order of mV) where they can be processed bycircuitry (e.g., an analog-to-digital converter (ADC)) in the NMR system100 and analyzed by the controller 110. In a conventional approach, withreference to FIGS. 2A and 2B, the NMR system 100 includes an amplifierto generate a fixed, constant gain 202 for amplifying the received NMRsignal 118. The fixed gain 202 is typically tuned based on the materialproperties of the tested sample 104 and/or the NMR coil 102. Afteramplification, the NMR signal 204 still represents an exponential decayin the time domain; thus, the damped portion 206 of the amplified NMRsignal 204 may still be too weak to be processed and/or analyzed.Although it may be possible to increase the fixed gain 202 to a levelthat allows sufficient amplification of the damped portion 206, theincreased fixed gain 202 may cause saturation of the less-damped portion208 of the amplified NMR signal 204; analysis of a saturated (andthereby distorted) NMR signal may provide inaccurate information aboutthe sample 104. As understood herein, the less-damped portion mayinclude the oscillations received within a decay time or a half-lifetime of the NMR signal. Alternatively, the less-damped portion mayinclude the initially received several oscillations (e.g., 2, 5, 8 or 10oscillations) of the NMR signal. The damped portion typically includessome or all oscillations of the NMR signal outside the less-dampedportion.

To overcome this challenge, in various embodiments, a time-dependentamplifier gain is applied to the received NNR signal 118 so as toprovide variable amplification to the damped and less-damped potionsthereof. Preferably, the amplifier gain applied to the damped portion islarger than that applied to the less-damped portion; this advantageouslyamplifies the damped portion to a level suitable for processing andanalysis while avoiding saturation of the less-damped portion. Forexample, referring to FIGS. 3A and 3B, an amplifier gain 302 thatgradually, continuously increases over time with an exponential gainprofile may be applied to the received NMR signal 118. As a result, themeasured NMR decay signal 118 is effectively linearized by theexponential gain (as shown by the amplified signal 304). This approachthus allows the damped signal to be amplified to a desired level (theamplified damped portion 306) suitable for processing without distortingthe less-damped portion 308. In one embodiment, the time constant of theexponential gain profile 302 is determined based on the expectedcharacteristic time T₂ of the NMR echo signal. For example, the timeconstant of the exponential profile 302 and the expected characteristictime T₂ may have the same order of magnitude; in some embodiments, thedifference therebetween may be smaller, e.g., less than 100%, 50%, or10%. Because the characteristic time T₂ is material-specific, itsexpected value can be acquired empirically via any suitable approachprior to the NMR measurements and/or from known literature or a lookuptable.

Significant amplification of the damped portion 306 such that it can beincorporated in the detected NMR signal for data analysis may haveseveral advantages. For example, the NMR signal in the time domain maybe converted using a Fourier transform to a spectrum in the frequencydomain. Incorporation of the damped signal 306 may yield a betterresolution in the NMR spectrum. In various embodiments, the amplifiedNMR signal 304 is subsequently processed (e.g., by an ADC) and analyzedby the controller 110 to obtain an image or other information (e.g., thecomposition) of the sample 104. For example, to acquire the actualcharacteristic time T₂ of the received NMR signal 118, in oneimplementation (with reference to FIG. 3C), the controller 110 firstcomputationally applies to the amplified, linearized signal 304 a gainfunction 312 inverse to the exponential profile 302 and then, based onthe resulting signal, computes the actual characteristic time T₂ of thereceived signal 118. By taking into account the time-dependent profileof the amplifier gain to provide accurate information about the sample104, this approach allows the received NMR signal 118 (in particular,the damped portion thereof) to be sufficiently amplified to a levelsufficient to facilitate processing by NMR circuitry (e.g., the ADC) andsubsequent analysis by the controller 110.

In various embodiments, the amplifier gain is increased in a discretemanner over time to lessen the design burden associated with theamplifier. For example, referring to FIG. 4A, the amplifier gain may beincreased in a step-wise manner; the gain difference between adjacentsteps may be uniform or may vary among steps. In addition, the timeduration of each step may be uniform or may vary. For example, the timeduration of each step may be the same—i.e., Δt₁=Δt₂=Δt₃=Δt₄, whereas thegain increase between steps may be gradually decreased—i.e.,ΔA₁>ΔA₂>ΔΔA₃. In one embodiment, the overall profile 402 of the discretegains is approximated as an exponential increase. This gainconfiguration may effectively linearize the received NMR signal 118 in apiece-wise manner (as exemplified by the amplified signal 404 in FIG.4B); this also generates a desired amplification of the measured NMRsignal 118—i.e., the damped portion of the received NMR signal isamplified to a greater degree than the less-damped portion, allowing thedamped signal to be processed without saturation of the less-dampedsignal. Again, upon receiving the amplified signal 404, the controller110 may apply thereto an inverse gain function 406 (with reference toFIG. 4C) so as to compute the actual characteristic time T₂ of thereceived signal 118.

It should be noted that the configurations of the amplifier gainprofiles 302, 402 described herein are for illustration only; thepresent invention is not limited to such configurations. One of ordinaryskill in the art will understand that variations are possible, so longas the time-dependent amplifier gain applied to the damped portion islarger than that applied to the less-damped portion; any amplifier gainprofiles satisfying this condition are thus within the scope of thepresent invention.

In various embodiments, the amplifier gain applied to the received NMRsignals 118 is dynamically adjusted during the NMR measurements. Forexample, referring to FIG. 5, the transmitter 112 may generate anddeliver to the coil 102 a pulse sequence 500 consisting of multiplepulses 502-506. At the end of the first pulse 502, the coil 102 may bedisconnected from the transmitter 102 and connected to the receiver 114,via, for example, a switch, for receiving an echo signal 508 from thesample 104. Upon receiving the echo signal 508, the controller 110 mayadjust the time-dependent amplifier gain based on an expectedcharacteristic time T_(2exp) associated with the tested sample inaccordance with the approach described above. Alternatively, theamplifier gain may be a fixed value that is sufficient to amplify thedamped portion for signal processing without saturating the less-dampedportion of the NMR signal. The controller 110 then computes the actualcharacteristic time T₂ associated with the measured NMR echo signal 508by taking into account the amplifier gain (e.g., by applying an inversegain function as described above). In addition, the controller 110 mayconvert the signal 508 to the frequency domain for spectral analysis.

In some embodiments, after the echo signal 508 is received, the coil 102is disconnected from the receiver 114, and reconnected to thetransmitter 112 for transmitting a subsequent pulse 504 to the sample104. Again, the receiver 114 may be reconnected to the coil 102 at theend of the pulse 504 in order to detect the echo signal 510. The newlyreceived echo signal 510 may be amplified with an exponential gain whosetime constant is determined based on—e.g., matches or is scaled withrespect to—the measured characteristic time T₂ of the previouslydetected NMR echo signal 508. Similarly, the characteristic time T₂ ofthe current echo signal 510 may determine the time constant of theexponential gain applied to the next received echo signal 512. In someembodiments, the amplifier gain applied to the currently received NMRsignal is determined based on multiple preceding measurements. Forexample, the time constant of the exponential gain applied to the echosignal 512 may determine based on an average of the time constants ofthe signal 508 and signal 510. By dynamically adjusting the amplifiergain applied to the received NMR signals (particularly the dampedportions) based on the “real-time” measurements of the sample 104, eachreceived NMR signal may be optimally amplified to provide accurateinformation about the sample. In addition, this approach may relax theperformance requirements of the ADC that often is used to digitize theamplified signals by optimizing the signal amplitudes, thereby reducingthe necessary dynamic range of the ADC.

The time-dependent amplifier gain described above may be provided byamplifier stages implemented in the NMR system 100. Referring to FIG.6A, in various embodiments, an exemplary NMR transceiver 602 isimplemented in a complementary metal-oxide semiconductor (CMOS)integrated circuit (IC) chip. The transceiver 602 is generally coupledto the NMR coil 102 via an impedance-matching network 604. Circuitcomponents of the transceiver 602 typically include a pulse-sequencegenerator 606, a power amplifier (PA) 608, three amplifier stages610-614 for amplifying the received NMR signals from the sample, andmixers 616 for down-converting the received signals to a lower frequencyin order to relax the requirements on the ADC. In addition, a switch 618may be included between the PA 608 and the impedance-matching network604; this switch 618 can connect the RF coil 102 to the transmitterportion 620 or receiver portion 622 of the transceiver 602 as describedabove so as to transmit an RF signal to the sample 104 and receive anecho signal therefrom.

In various embodiments, the three amplifier stages 610-614 include twolow-noise amplifiers (LNA) 610, 612 that set the overall noiseperformance of the receiver 622 and a variable gain amplifier (VGA)(such as a programmable gain amplifier, PGA) 614 for amplification ofthe NMR signals detected by the coil 102. The VGA 614 may apply atime-dependent amplifier gain to individual NMR signals and/or adynamically adjusted amplifier gain profile to a series of received NMRsignals in a manner as described above. In some embodiments, thepulse-sequence generator 606 includes the gain setting to be used by theVGA 614 during acquisition of the NMR signals. For example, referring toFIG. 6B, the pulse-sequence generator 606 may generate a pulse sequence630 having pulses 632-636; the amplitudes of the pulses 632-636 may beincreased in a step-wise manner and the amplitude difference between twoadjacent pulses may be an integer of a regular step size of the VGA 614.For example, the step size of the VGA 614 may be 0.5 decibel (dB); theamplitude difference ΔA₁ between pulses 632 and 634 and ΔA₂ betweenpulses 634 and 636 may be set by the sequence generator 606 to be 1 and1.5 dB, respectively. As a result, after the VGA 614 applies anamplifier gain 638 to the received echo signal 640 corresponding to thepulse 632, the VGA 614 may decrease the amplifier gain by twosteps—i.e., 1 dB to amplify the echo signal 644 corresponding to thepulse 634. Similarly, after amplification of the signal 644, the VGA 614may decrease the amplifier gain by another three steps—i.e., 1.5 dB toamplify the echo signal 646 corresponding to the pulse 636.

FIG. 7 is a flow chart 700 illustrating an approach for dynamicallyadjusting the amplifier gain of an NMR transceiver during NMRmeasurements in accordance with various embodiments of the invention. Ina first step 702, the NMR transceiver 602 transmits a first pulse to anNMR coil 102 surrounding a sample 104 and receives a first echo NMRsignal therefrom. Optionally, the received NMR echo signal may beamplified using a fixed gain or a time-dependent gain (e.g., anexponentially increasing gain or a discretely increasing gain). Thevalue of the fixed gain or a characteristic of the time-dependent gain(e.g., the time constant of the exponential gain or the increase of thediscrete gains) may be determined based on the expected materialproperty of the sample and/or the coil (which may be obtainedempirically prior to the measurements using any suitable techniqueand/or from known literature or a look-up table). In a second step 704,the echo signal is processed by the NMR circuitry and analyzed by acontroller to determine its characteristic time T₂. If the echo signalis amplified, the controller may first apply to the amplified signal again function inverse to the applied amplifier gain, and then analyzethe resulting signal to compute the actual characteristic time T₂ of thereceived NMR signal. In a third step 706, the NMR transceiver 602transmits a second pulse to the NMR coil 102 and subsequently receives asecond echo NMR signal from the sample. In a fourth step 708, thereceived second echo signal is amplified using a second time-dependentamplifier gain; the characteristic of the second time-dependent gain(e.g., the time constant of the exponential gain or the discreteincrease of the gain) is determined based on the characteristic time T₂of the first received echo signal. Subsequently, the amplified secondecho signal is processed to determine its characteristic time T₂ (in afifth step 710). Steps 706-710 may be iteratively performed throughoutthe entire NMR measurements. As a result, the amplitudes of the receivedNMR signals are dynamically amplified via adjustment of the amplifiedgain in accordance with the characteristic time of previously receivedNMR signal. This approach thus enables the damped portions of thereceived NMR echo signals to be amplified with a greater degree(compared with amplification using a fixed-gain amplification) based onthe actual, real-time measurement of the sample. This allows the NMRsignals to be accurately analyzed, which may improve the spectralresolution of the NMR spectrum. It should be noted that the approach 700described above is for illustration only; any suitable approach todetermining the characteristic of the amplifier gain to be applied tothe currently received signal based on the preceding signal(s) arewithin the scope of the present invention. For example, thecharacteristic of the amplifier gain applied to the current NMR signalmay be determined based on an average of two or more characteristictimes associated with the preceding echo signals.

In general, functionality for analyzing the received NMR signals (suchas computing the characteristic times of the received signals andapplying an inverse gain function to the signals) and operating the NMRcircuitry (such as causing the transceiver to transmit a sequence ofpulses and receive echo signals from the sample and using VGA to apply afixed or time-vary gain to the received signals) as described above,whether integrated within a controller of the NMR system, or provided bya separate external controller, may be structured in one or more modulesimplemented in hardware, software, or a combination of both. Forembodiments in which the functions are provided as one or more softwareprograms, the programs may be written in any of a number of high levellanguages such as PYTHON, FORTRAN, PASCAL, JAVA, C, C++, C#, BASIC,various scripting languages, and/or HTML. Additionally, the software canbe implemented in an assembly language directed to the microprocessorresident on a target computer (e.g., the controller); for example, thesoftware may be implemented in Intel 80×86 assembly language if it isconfigured to run on an IBM PC or PC clone. The software may be embodiedon an article of manufacture including, but not limited to, a floppydisk, a jump drive, a hard disk, an optical disk, a magnetic tape, aPROM, an EPROM, EEPROM, field-programmable gate array, or CD-ROM.Embodiments using hardware circuitry may be implemented using, forexample, one or more FPGA, CPLD or ASIC processors.

In addition, the term “controller” used herein broadly includes allnecessary hardware components and/or software modules utilized toperform any functionality as described above; the controller may includemultiple hardware components and/or software modules and thefunctionality can be spread among different components and/or modules.

Certain embodiments of the present invention are described above. It is,however, expressly noted that the present invention is not limited tothose embodiments; rather, additions and modifications to what isexpressly described herein are also included within the scope of theinvention.

What is claimed is:
 1. A method of amplifying an NMR signal in responseto a transmitted NMR pulse, the method comprising: estimating acharacteristic time associated with the NMR signal; transmitting the NMRpulse to a sample and receiving the NMR signal therefrom; and applying atime-dependent amplifier gain to the received NMR signal based at leastin part on the estimated characteristic time.
 2. The method of claim 1,wherein the characteristic time is estimated based on a previous NMRmeasurement of the sample.
 3. The method of claim 1, further comprisingcomputing a second characteristic time associated with the amplified NMRsignal.
 4. The method of claim 3, wherein computation of the secondcharacteristic time comprises applying an inverse gain function to theamplified NMR signal.
 5. The method of claim 3, further comprisinganalyzing at least a portion of the amplified NMR signals occurringafter passage of the second characteristic time.
 6. The method of claim1, further comprising: transmitting a second NMR pulse to the sample andreceiving the second NMR signal therefrom; and applying a secondtime-dependent amplifier gain to the second NMR signal based at least inpart on the second characteristic time.
 7. The method of claim 1,wherein the time-dependent amplifier gain comprises an exponential gainprofile.
 8. The method of claim 7, wherein a time constant of theexponential gain profile and the characteristic time are within an orderof magnitude.
 9. The method of claim 1, wherein the time-dependentamplifier gain comprises a series of discrete gain increases.
 10. Themethod of claim 9, wherein a profile of the series of discrete gainincreases comprises an exponent.
 11. An NMR apparatus comprising: atransceiver for transmitting an NMR pulse to a sample and receiving anNMR signal therefrom; and a controller configured to: (i) estimate acharacteristic time associated with the NMR signal; (ii) cause thetransceiver to transmit the NMR pulse to the sample and receive the NMRsignal therefrom; and (iii) cause a time-dependent amplifier gain to beapplied to the received NMR signal based at least in part on theestimated characteristic time.
 12. The NMR apparatus of claim 11,further comprising a variable-gain amplifier for applying thetime-dependent amplifier gain to the received NMR signal.
 13. The NMRapparatus of claim 12, wherein the variable-gain amplifier is aprogrammable-gain amplifier.
 14. The NMR apparatus of claim 11, furthercomprising a pulse-sequence generator having a gain setting associatedwith gains generated by the variable-gain amplifier.
 15. The NMRapparatus of claim 14, wherein the pulse-sequence generator generates aplurality of NMR pulses, wherein an amplitude difference betweenadjacent pulses is determined based at least in part on a step size ofthe gains generated by the variable-gain amplifier.
 16. The NMRapparatus of claim 11, wherein the controller is further configured toestimate the characteristic time based on a previous NMR measurement ofthe sample.
 17. The NMR apparatus of claim 11, wherein the controller isfurther configured to compute a second characteristic time associatedwith the amplified NMR signal.
 18. The NMR apparatus of claim 17,wherein the controller is further configured to computationally apply aninverse gain function to the amplified NMR signal.
 19. The NMR apparatusof claim 17, wherein the controller is further configured to analyze atleast a portion of the amplified NMR signals occurring after passage ofthe second characteristic time.
 20. The NMR apparatus of claim 11,wherein the controller is further configured to: cause the transceiverto transmit a second NMR pulse to the sample and receive the second NMRsignal therefrom; and cause a second time-dependent amplifier gain to beapplied to the second NMR signal based at least in part on the secondcharacteristic time.
 21. The NMR apparatus of claim 11, wherein thetime-dependent amplifier gain comprises an exponential gain profile. 22.The NMR apparatus of claim 21, wherein a time constant of theexponential gain profile and the characteristic time are within an orderof magnitude.
 23. The NMR apparatus of claim 11, wherein thetime-dependent amplifier gain comprises a series of discrete gainincreases.
 24. The NMR apparatus of claim 23, wherein a profile of theseries of discrete gain increases comprises an exponent.
 25. A method ofdynamically varying gain amplification of received NMR signals inresponse to transmitted NMR pulses, each received NMR signalcorresponding to a transmitted NMR pulse, the method comprising: (i)transmitting a first one of the NMR pulses to a sample and receivingtherefrom a first one of the NMR signals; (ii) determining acharacteristic time associated with the first one of the NMR signals;(iii) transmitting a second one of the NMR pulses to the sample andreceiving therefrom a second one of the NMR signals; and (iv) applying atime-dependent amplifier gain to the received second one of the NMRsignals based at least in part on the determined characteristic time.26. An NMR apparatus comprising: a transceiver for transmitting NMRpulses to a sample and receiving NMR signals therefrom, each NMR signalcorresponding to a transmitted NMR pulse; and a controller configuredto: (i) cause the transceiver to transmit a first one of the NMR pulsesto the sample and receive therefrom the first one of the NMR signals;(ii) determine a characteristic time associated with the first one ofthe NMR signals; (iii) cause the transceiver to transmit a second one ofthe NMR pulses to the sample and receive therefrom the second one of theNMR signals; and (iv) cause the transceiver to apply a time-dependentamplifier gain to the received second one of the NMR signals based atleast in part on the determined characteristic time.